1. Field of the Invention
This invention relates to electropneumatic positioners for pneumatically actuated valves, and more particularly, to an apparatus and method for electrically overriding operation of an electrically controlled electropneumatic positioner.
2. Background Information
Throughout this application, various publications, patents and published patent applications are referred to by an identifying citation. The disclosures of the publications, patents and published patent applications referenced in this application are hereby incorporated by reference into the present disclosure.
Modern process plants contain innumerable operating components. These components are tied together to form systems controlled by instrumentation and control systems containing sensors and controllers. The instrumentation and control systems on such plants not only serve to control the functions of the various components in order to achieve the desired process conditions, but they also provide the facility to safely modify or discontinue operation of all or a portion of the plant's systems in order to avoid an unsafe situation or condition.
One of the means by which such safety systems function is by the securing or diverting of the supply of a certain process or control fluid, or the supply of motive power to a plant system or component of a plant system. Such systems often utilize pneumatically operated valves. One of the means by which the safety functions can be accomplished is through the use of solenoid operated valves connected in series between the pneumatic control source and the pneumatically operated valve.
In operation, the pneumatic valves or actuators are isolated from the pneumatic control source and pressure within the actuator is vented off when the solenoid of the solenoid valve is repositioned (e.g., de-energized). In this manner, the pneumatic actuator may return to a configuration designated for safety. An example of a safety system which utilizes solenoid valves within a pneumatic system is disclosed in U.S. Pat. No. 5,665,898, to Smith et al. An example of a typical plant system including a pneumatic actuator and a solenoid valve safety device is shown in FIG. 1.
Referring to FIG. 1, conventional electropneumatic positioners 10 typically include an electronic position controller such as a microprocessor 12 which controls operation of a pneumatic valve relay 14 pursuant to signals 32 received from a factory automation system or other computer network 34. An example of such a positioner 10 is Model No. SRD991-BFMS2FAA available from The Foxboro Company of Foxboro, Mass., USA. Relay 14 in turn directs pneumatic fluid (air or other gas) along a conduit 15 through a solenoid valve 16 to a pneumatic actuator 18. Actuator 18 includes a stem 20 which is movable in response to the pneumatic pressure to actuate (i.e., open or close) a fluid process control valve 22. One or more sensors 24 may be utilized to detect the actual position of stem 20 to provide position feedback to the controller 12 as shown at 35. Any difference between the system setpoint signal 32 and the position feedback signal 24 then may be determined and corrected for by the position controller 12.
As shown, the solenoid valve 16 is included as a safety device to quickly exhaust the pneumatic pressure in the event of a malfunction etc., to move the actuator 18 to its safe configuration and thus effectively override the controller 12. For example, the solenoid valve 16 may be utilized to exhaust the pneumatic fluid (i.e., air), in the event the pressure of the pneumatic supply 40 drops below a predefined limit, such as may occur during a plant shutdown due to compressor fault, etc., to dispose actuator 18 in its depressurized (i.e., safe) position.
While the use of solenoid valve 16 may provide sufficient safety in many applications, it is not without drawbacks. For example, provision and installation of the solenoid valve 16 and pneumatic conduit associated therewith disadvantageously increases the material and labor (i.e., installation) cost of the electropneumatic positioner 10. Further, solenoid valve 16 typically operates in a binary fashion i.e., the valve is operable between fully open and fully closed positions. This aspect thus tends to require that the flow through valve 22 be completely discontinued (rather than being partially reduced) when in the safety configuration. Moreover, the solenoid valve 16 should be tested periodically to help ensure proper operation thereof. Such testing thus tends to disadvantageously generate frequent interruption of the flow through valve 22.
Thus, a need exists for an improved device and method for selectively overriding control signals to a pneumatic actuator.